1. Field of the Invention
The present invention relates to DC-DC power regulators, and more particularly to a system and method for output current sharing among multiple DC-DC PWM converters using droop control feedback.
2. Description of the Related Art
In many electronic devices and systems, including computer systems and peripherals, it is often desired to combine power from two or more power sources. Multiple power supplies are desired, for example, when a single source is insufficient or when redundancy is desired or otherwise required. A computer system typically has multiple rail voltages or voltage source levels, such as, for example, 12 Volts (V), 5 V, 3.3 V, etc., where each rail is capable of providing a predetermined maximum amount of power (or current) according to a corresponding specification. If a device requires more power than any single voltage source can provide, the device combines power from multiple rails or sources. For example, plug-in peripheral cards, such as graphics cards or the like, receive power from one or more rails available on the expansion slot. Each rail of each slot typically has a specified voltage level and a maximum current rating, so that if the peripheral device requires more power than any one rail can provide within specified limits for that slot, the device must combine power from multiple rails. In some cases, if the available power from the slot is insufficient, an external power source, such as a disk drive power connector or the like, must be employed to provide the additional power.
A buck converter, also known as a step-down converter or down-converter, is a DC-DC converter that converts a higher voltage input (which may be from an unregulated source, such as a battery or the like), to a regulated lower voltage output. For example, a buck converter may be used to convert a higher rail voltage (e.g., 12 V, 5 V, 3.3 V, etc.) to the appropriate voltage level required by the device (e.g., 1.5 V). If any one available rail is unable to supply all of the power necessary, multiple buck converters are coupled in parallel so that the necessary power is provided from multiple rails. It is intended that each converter provide a proportionate share of the total current supplied to the device, especially under full load conditions.
The techniques for paralleling power converters is often inefficient. The outputs of multiple converters are often combined at an ORing or summing junction or the like, resulting in a loss of power through the ORing or summing junction devices. For example, the outputs may be combined using a resistor sharing network, resulting in power loss in each of the resistors. Alternatively, the outputs are coupled together in an OR configuration using ORing diodes or the like, resulting in power loss in each of the diodes. Such paralleling techniques do not always provide the desired level of current sharing among the converters, particularly during full-load conditions. Some converters may dominate or overwhelm other converters during certain load conditions, such as full-load or transient load conditions (sudden change in load, such as sudden load increase or decrease).
It is desired to provide more efficient current sharing among multiple power source rail voltages, especially under full load conditions. It is also desired to set the relative power sharing between the power sources regardless of the particular rail voltage levels provided to each.